Honda C90 – carburettor – how it works

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When I first was first researching what I needed to do to get my bike to run I quickly realised I would need to clean the carburettor in case it had become clogged up with old fuel.

I agonised over this as I knew it was a critical part and – despite having watched various videos of people dismantling and cleaning them – I still had no idea what all the strangely named parts did. Eventually I just had to get on with it and take it apart – needless to say it all went back together without drama.

After cleaning the carburettor the bike would at least run, but it did not idle well and was hard to start, and it soon dawned on me that I would not have much chance of understanding what the problem was without first trying to understand how the carburettor actually worked.

This will be the first of a series of posts where I revisit some of the mechanical work I did on the engine, nearly all of which I did by blindly following instructions rather than understanding what I was doing or why it was necessary.

The trouble with this type of approach is that it is very easy to miss the things you get wrong and, in my case at least, I find it almost impossible to remember what to do fix and adjust mechanical things I can understand why it is needed,

The next few posts will be my attempt to summarise what I have learned out carburettors from a combination of reading and taking things apart and staring at them.

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The (pre-clean) insides of my carbuerttor

As the piston in the engine moves downwards, air is sucked into the carburettor and – as the air moves through the body of the carburettor – the channel inside narrows, and this causes the air to speed up and the pressure in this section to be reduced.

HOW IT WORKS

The fuel, which is held in a bowl at the base of the carburettor, is pulled into the low pressure area where it mixes with the air on the way through to the chamber.

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That’s it! This is essentially all the carburettor does. But what is going on? With apologies in advance to any scientists who might be reading this, here is an explanation:

The carburettor works because of the Venturi effect (named after Giovanni Venturi). This is the name for the reduction in pressure that occurs when a fluid (i.e. a liquid or a gas) flows through a constricted section in a pipe.

As the air passes through the narrowest point it has to speed up to get the same volume of air through the narrower gap and, as it accelerates, the pressure drops.

Luckily we don’t need to understand the mathematical relationship between air speed and pressure, we just need to know that fast moving air has lower pressure.

One way to think about what is happening is that the air is being stretched so that it contains fewer molecules per unit of volume and is therefore less dense, creating less pressure.

The relationship between fluid velocity and pressure was first observed by 18th C scientist called Daniel Bernoulli). Here is a proper scientific explanation for those that might be interested.

Armed with this rudimentary understanding of fluid dynamics we can try and figure out what the various parts of the carburettor are for.

We will be looking at the carburettor that came with my c90 z2, a PB25A. It was made by Keihin, who have been making carburettors for Honda since the late 1950s, and is one of several designs used on the c90 over the years.

here is a schematic of the carb:

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it is not as complicated as it first appears, honest. We look at what all the bits do in a subsequent posts.

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